The principle for the measurement of carbon equivalent involves monitoring a ternary, or a pseudobinary system, comprising the elements of iron, carbon and silicon at the initial thermal arrest temperature (primaly crystal to be liquidus) when the melt of cast iron solidifies to make it carbon equivalent. Though carbon equivalent is expressed in various ways the most general definition is the total percent of carbon plus one-third (Si% plus P%). A further application involved in carbon equivalent measurements are measuring eutectic temperature, interrelating the temperature with liquid, liquid plus solid, and solid determining, the silicon and carbon contents, and estimating the physical and mechanical properties of the cast iron as a function of the time until solidification of the melt. It is known that according to the application, the numerical values thus obtained are corrected and analyzed depending on the modification of practical profile, material and the like of the casting. Further, it is possible to know the state of melt accurately and more quickly and field-like than any other analyzing method by means of a cup which is manufactured by a known technique and pre-arranged before the pouring of the melt to allow the melt to be administered before casting. In case the constituents and various properties of the molten cast iron differ from those intended a suitable pretreatment can be carried out. These known techniques are disclosed in U.S. Pat. No. 3,267,723 and Japanese Patent No. 820,206 based thereon.
Compared with the time when said known techniques were proposed, a wider range of casting materials are available in the present casting industries. High technology has provided compacted graphite iron (so-called CV cast iron) or austempered ductile iron (so-called ADI cast iron) and cast iron alloys have been improved and developed which require an accurate and a quicker technique of administering the in-situ furnace front melt. That is, iron and carbon system is a binary system whereas iron, carbon and silicon system is a ternary system. In the binary system the eutectic temperature is constant whereas in the ternary system it is maximum or minimum, and conditions have become diverse such that cast irons are greatly affected by the silicon content, and according to the additive elements for cast iron alloys the eutectic temperature rises for some elements and is lowered for some others. A further complicated phenomenon is that in the solidification of the basic elements iron, carbon and silicon of ternary system there are two equilibriums of completely stable solidification of iron-carbon (graphite)-silicon system and metastable solidification of iron-carbon (cementite)-silicon system.
Additionally, as the cooling rate changes or the additive elements differ, the two equilibriums alternately occur in the same melt in some occasions. Such a multi-equilibrium problem causes the complication of solidification of cast iron system. Under the existing circumstances where operation by cupola is gradually shifting these days to the melting by electric furnace because of the control to the environmental contamination, serious problems are presented not only to steels but also to cast irons. The problems include not only the characteristic change of the melt caused with time after melting and the simple change of the constituents of the melt but also for the oxygen content, oxide or solved oxygen content in the melt.
This makes known thermal analysis methods unsatisfactory. It is therefore necessary to accurately measure not only the primarily crystallized temperature but also the eutectic temperature. Thus, in order to measure the cooling curve in a safe white pig iron state (iron and cementitc system), it is necessary to measure the semi-stable solidification as a requisite condition, not only for the cooling curve, but also for samples for mechanical analysis such as in emission spectrum analysis method, X-ray analysis method or the like.
The inventor of this invention has performed extensive research in an attempt to solve the above problems. Known methods of adding graphitization-hindering elements such as tellurium, bismuth and boron as metastable solidification promoters according to prior art include one of adding, as a paint for chill wash or the like, metal tellurium powder or the like in a measuring cup. This method is based on the above-referred patent inventions. However, it is questionable in those inventions whether said elements are always accurately added in a constant proportion, and it is doubtful whether correct primaly crystallized and eutectic temperatures are always obtained from the samples of melts of cast irons. Referring to a method of obtaining a cooling curve, Japanese Patent No. 820,206 describes in its claims the addition of bismuth, boron, cerium, lead, magnesium and tellurium, and compounds and mixtures thereof into the melt as stabilizer. However, cerium and magnesium are spherification reaction agents and spherification stabilizers for typical ductile cast irons, and they disturb the equilibrium state of metal-stable solidification of iron-cementite systems necessary to make the measurement of respective primaly crystallized and eutectic temperatures. It is naturally not until the measurement of the equilibrium state that the primaly crystallized and eutectic temperatures are measured. Cerium and magnesium necessarily pass through deoxidation, desulfuration and decarburization processes before the spherification of graphite, and it will be clear it is inconvenient to use elements which obtain a decarburization action, in measuring the carbon equivalent and carbon content.